The invention relates to hearing aids, and more particularly relates to directional hearing aids. In its most immediate sense, the invention relates to directional hearing aids of the In-The-Ear (xe2x80x9cITExe2x80x9d) type. (Included in the ITE type are so-called xe2x80x9chalf shellxe2x80x9d aids, which are smaller than full size ITE aids but are larger than canal aids and Completely-In-Canal or xe2x80x9cCICxe2x80x9d aids.)
Conventional hearing aids have only one omni-directional microphone, so the patient can hear sound from all directions around his or her head. This omnidirectionality impairs the patient""s ability to differentiate between e.g. the voice of a conversational partner and background noise (as from a crowd). For these reasons, directional hearing aids have been developed.
As conventionally implemented in ITE aids, a directional hearing aid has two small (EM size) omnidirectional microphones that are spaced apart by at least 6 mm and by at most 12 mm. An alternate implementation of an ITE directional hearing aid uses a capsule (sold under the D-MIC mark by Etymotic Research, Inc.) that contains an EM-size dual-input directional microphone and an EM-size omnidirectional microphone together with an appropriate electronic circuit. The inlets of the directional microphone are spaced apart by 4 mm.
In both instances, the directionality of the aid comes about because there is a phase shift of the sound pressure near the inlets of the two omnidirectional microphones (and, likewise, near the two inlets of the directional microphone). Sound will reach one inlet before it reaches the other, and the resulting phase shift in combination with an internal delay of the microphone will determine the polar response of the microphone.
These two known directional ITE implementations share a significant reduction of the signal-to-noise ratio, relative to that of a conventional non-directional ITE aid. Two factors significantly contribute to this problem.
The first factor is that a directional microphone with close spacing between the inlets (of two omnidirectional microphones or of the two inlets of a dual-input microphone) has a pronounced (6 dB/octave) rolloff at low frequencies. (This rolloff comes about because lower-frequency sounds have longer wavelengths. As a result, for a particular spacing, the phase shift of the sound pressure near the inlets diminishes with decreasing frequency of the incident sound.) This rolloff reduces the sensitivity (and therefore the signal-to-noise ratio) of the aid, and requires significant electrical equalization. Such equalization amplifies the low-frequency noise, and interferes with the patient""s hearing in quiet situations.
The second factor is that all other things being equal, smaller microphones generally have smaller signal-to-noise ratios. This is because a smaller microphone must have a smaller membrane, which makes the microphone less sensitive since sensitivity increases with membrane size. In quiet situations, smaller (EM-size) directional microphones can be unacceptably noisy.
To address the problem of excessive noise in quiet situations, both types of ITE hearing aids are provided with a patient-operable switch. This switch puts the aid in an omnidirectional mode when the internal noise in the directional mode becomes unacceptable to the patient. Such a switch adds to the cost of the components required to manufacture the aid, and also takes up valuable space (xe2x80x9creal estatexe2x80x9d) on the faceplate. Because of the real estate required by the switch and the two separate microphones that must be spaced apart by at least 6 mm, certain patients e.g. those with small earsxe2x80x94may be unable to be fitted with directional hearing aids. Alternatively, such patients may be forced to accept larger ITE aids instead of xe2x80x9chalf shellxe2x80x9d aids, which are less conspicuous and are therefore cosmetically preferable.
Additionally, if a directional ITE hearing aid is constructed using two omnidirectional microphones, the microphones must be well matched in respect of frequency response etc., which increases the costs of components and assembly.
It would be advantageous to provide a directional hearing aid of the ITE type where the internal noise is not substantially higher than in a conventional ITE aid. Such a directional aid would not require a patient-operable mode switch, would be less expensive to manufacture, and would use less real estate on the faceplate.
In accordance with the invention, the two small (conventionally, EM size) individual microphones that are conventionally used in an ITE aid are replaced by a bigger (advantageously, EL size) conventional dual-inlet microphone (similar, but not identical, to that presently manufactured by Knowles Electronics, Inc. as Model EL). And, in further accordance with the invention, the inlets of the microphone are connected to two spaced-apart ports in the faceplate of the aid via two outwardly diverging channels that are located in the faceplate. As a result of this structure, the ports are spaced sufficiently far apart so that the aid can be directional with maximum possible signal-to-noise ratio, without taking up valuable real estate on the faceplate of the aid.
Although dual-inlet microphones are conventionally used to make directional hearing aids of the Behind-The-Ear (xe2x80x9cBTExe2x80x9d ) type, the inventor is unaware of any use of such a microphones to replace the two individual microphones previously used in ITE applications. Now that this use has taken place, it is evident that the invention produces new and unexpectedly advantageous results.
One such result is that the microphone is so quiet that a patient-operable mode-adjustment switch is not required; the aid can be maintained in the directional mode without unacceptable noise. This comes about because of the inherent characteristics of a dual-inlet EL type microphone. (These characteristics will be discussed below.) Because the switch is not required, the cost of components is reduced and valuable real estate on the faceplate is made available for other uses.
Additionally, the invention substantially reduces the costs of components and the labor required to assemble the hearing aid. The cost of a single dual-inlet microphone is substantially less than the cost of two individual microphones having matched characteristics, and it requires less labor to connect one microphone to the hearing aid electronics than to so connect two microphones (and a mode-selection switch).
Furthermore, because a dual-inlet microphone is less bulky than two individual microphones, the savings in faceplate real estate make it possible to build a directional aid in a smaller volume. As a result, more patients can be provided with a directional ITE aid, and some patients can even be provided with a xe2x80x9chalf shellxe2x80x9d aid.